1. Field of the Invention
This invention relates to a method for raising small marine echinoderms and bivalve mollusks which need to settle on a substrate to develop into juveniles and ultimately into adults, and particularly to a method for culturing sea urchins from settling to maturity or any stage from metamorphosis to market.
2. Description of the Related Art
The increasing demand for aquatic animals including echinoderms and mollusks as a source of food, as well as concerns regarding over-harvesting and potential contamination from pollution, have led to the growth and expansion of the aquaculture industry.
Echinoderms include starfishes, sea urchins, and sea cucumbers, having an internal calcareous skeleton and often covered with spines. Bivalve mollusks include oysters, clams, scallops, and mussels, having a shell consisting of two hinged valves. Spawning of mature marine animals results first in the development of the embryo. The embryo develops to a larval stage. Initially, the larvae are free swimming. After a time, the larvae metamorphose into a juvenile stage, and are capable of crawling and attaching to a substrate. After settling on a substrate, the juveniles are allowed to develop and are finally harvested when they reach a suitable size.
The juvenile stage of the green sea urchin may be broken up in four stages, i.e. post-metamorphosis, neo-juvenile, early juvenile and juvenile. The post-metamorphosis stage initiates immediately following metamorphosis and lasts until the development of the mouth is complete and feeding can commence (5–14 days). During the neo-juvenile stage, the individuals have a fully developed mouth but graze primarily on benthic diatoms and bacteria (about 30 days). In the early juvenile stage, the sea urchins have a diameter range of 1–15 mm and are able to consume macroalgae such as kelp, as well as mussels and other food sources. During the juvenile stage, the sea urchins have a diameter of over 15 mm, but have not reached sexual maturity. For additional details, see Devin et al., Development of Grow-Out Techniques for Juvenile Sea Urchins Strongylocentrotus droebachiensis, in Proceedings of the International Conference on Sea Urchin Fisheries and Aquaculture 2003, J. Lawrence, Ed., DEStech Publications, Inc. pp. 246–254.
Currently available aquaculture systems are generally classified as open or closed. Open systems are typically created by building a net-pen in a body of water, such as a bay, a lake or stream. Closed systems generally recirculate the water in a closed tank, the water being pumped from the tank through a treatment cycle and back into the tank.
For both open and closed aquaculture systems, the yield of harvested aquatic animals is always a significant factor impacting the commercial viability and profitability of farming. Thus, methods which produce commercially viable numbers of aquatic animals have been developed.
U.S. Pat. No. 4,080,930 discloses a method of rearing commercially desirable bivalve mollusks such as oysters, clams, mussels, and scallops under artificial conditions to any marketable size in substantially reduced time periods. Oyster spat (<0.5 mm) produced by known means are placed in growing tanks and fed at certain regular times, with certain species of algae in unusually large quantities, at unusually high algal cell concentrations, and maintained at unusually high temperatures until oysters reach the desired size. The tanks and oysters are meticulously cleansed and seawater or other source of salts and minerals is added at regular times during the rearing period.
U.S. Pat. No. 4,931,291 endeavors to achieve high yield of crustaceans and shellfishes to be cultivated, by employing unicellular algae whose cells are disrupted as the feeds for larvae.
U.S. Pat. No. 5,144,907 discloses a method of culturing giant scallops from the egg stage to the juvenile stage under controlled conditions, involving, as essential steps: after a pre-selected period of time of growing larvae in deep larval tanks, allowing the larvae to settle on bottom of the larval tanks in pre-selected lighting conditions and become spat, while periodically changing the water in the tanks and feeding the larvae with food; after a pre-selected period of time, brushing the spat out of the larval tanks, catching the brushed-out spat on screen trays; placing the spat contained on the screen trays in a culturing tank of selected water with the screen trays being stacked vertically therein; and periodically changing the water in the culturing tanks by inflow/outflow of water and monitoring food levels in the culturing tanks.
In order to produce high yields of aquatic animals, these methods attempt to establish optimum conditions related to the quality and quantity of food, feeding methods, temperature, and water. However, none of the methods proposes an inexpensive, durable and efficient habitat for economically producing commercially viable numbers of aquatic animals. For example, in U.S. Pat. No. 5,144,907, the larvae are allowed to settle on the bottom of the larval tanks until they develop to spat, and then are brushed out of the larval tanks, finally are put on screen trays as their substrate. This not only complicates the settling of spat and involves intensive labor, but also disturbs the growth of the aquatic animals, because farmers have to change the substrate and brush the spat out of the larval tanks. Moreover, the bottom of the tank and the screen trays do not provide a surface area for the spat like to settle on which fully utilizes the volume of the tank. Using mesh bags filled with bare mollusk shells in a tank as substrate has similar drawbacks.
U.S. Pat. No. 4,212,268 describes an aquaculture habitat for the rearing of post-larval crustaceans in which a cellular honeycomb structure, preferably of opposed cones, provides compartments for the animals and serves as the domicile core. Individual animals are inserted into the core compartments and the core is sealed with a screen or a mesh on either side of the core. This aquaculture habit is directed to raising small crustaceans whose carnivorous nature necessitates physical separation of the animals. This habitat still does not meet the need for a low cost, durable, efficient aquaculture habitat with a substrate having a large surface for raising the aquatic animals with high yield.
Thus, it is desired to have a durable, transportable, reusable and transportable aquaculture habitat which can be deployed at sea or in land-based tanks, and which utilizes space efficiently. It is also desired to have an inexpensive and light aquaculture habit made from simple, low cost materials. It is further desired that the habitat have a modular construction, so that the scale of the operation can be easily modified. Additionally, there is a need to provide convenience for the care and protection of the cultured animals.